Some ophthalmic procedures use highly precise diagnostic apparatus and/or surgical apparatus. To take advantage of the precision, it is important that the apparatus be accurately aligned with a subject's eye.
Various alignment apparatus are known. FIG. 1 is a schematic illustration of one conventional apparatus for axial alignment with a subject's eye. A first laser 10 and a second laser 20 are aligned so that the beams from the lasers intersect at a location L. The eye is brought to a longitudinal location L (in the z-direction) by observing an image of the light in the beams that is scattered by the subject's cornea C. The image is formed using a camera 50. When two spots R1 and R2 are observed by the camera, the subject's eye is either in front of or behind location L; and when a single spot is observed the subject's eye is proximate location L and assumed to be axially aligned. Such alignment apparatus have limitation on the degree of accuracy with which an instrument can be consistently aligned.